1. Field of Invention
The present invention relates to an apparatus and to a method for determining hematocrit.
2. Description of the Prior Art
Hematocrit is the percentage of packed red blood cells in a sample of whole blood. In the context contemplated for the present invention, hematocrit measurements are made in conjunction with a blood cell counter which includes a well-known conductivity sensor having an aperture and electrodes on either side thereof. A volume of a diluted blood sample is passed through the aperture, and conductivity changes due to blood cells passing through the aperture are monitored. Sensor pulses are produced representing the number of cells passing through the aperture, and each sensor pulse a peak representative of the volume of each cell (or group of cells resolved as a single cell). In earlier automatic hematology apparatus, a red blood cell count was provided in response to the number of pulses measured, and a calculation was made to determine means corpuscular volume (MCV). The hematocrit percentage was determined by multiplying the MCV times the red blood cell count and normalizing the results with respect to the volume passed through the sensor to obtain the hematocrit level as a percentage of the sample volume.
More recently, apparatus has been provided for direct determination of hematocrit. For example, U.S. Pat. No. 3,828,260 issued to Underwood on Aug. 6, 1974, discloses a hematocrit measuring apparatus in which a sensor pulse is stored and added to analog accumulation means for providing an analog level signal indicative of total cell volume of a sample. U.S. Pat. No. 3,812,425 issued to Miller on May 21, 1974, disclosed means for measuring hematocrit in which a sensor pulse peak is stored and in which the half-pulse level and threshold level of a trailing edge of the sensor pulse must be measured and processed to synchronize circuitry generating a pulse for addition to analog accumulation means which provide a level of analog voltage indicative of cell volume. Further, Miller's s apparatus must precisely time the accumulation, since volume measurements are not made. The Underwood apparatus, while providing satisfactory operation, utilizes the sensor pulse itself rather than intelligence generated therefrom for producing the hematocrit count. In the case of the Miller apparatus, many measurements must be made on each sensor pulse. Not all sensor pulses are substantially sinusoidal or bell-shaped. Depending on the orientation of a cell passing through the aperture or whether more than one cell is passing through the sensor at once, a sensor pulse may rise, fall, and rise again. Therefore, the number of direct measurements which must be made on the sensor pulse tends to decrease reliability of the hematocrit measuring means.